code: def infer_freq(y=None) -> Optional[str]:
      """Infer frequency string from the time series object.
  
      Parameters
      ----------
      y : Series, Panel, or Hierarchical object, or VectorizedDF, optional (default=None)
  
      Returns
      -------
      str
          Frequency string inferred from the pandas index,
          or `None`, if inference fails.
      """
      return None
  
file_path: sktime/sktime/utils/datetime.py
imports: import warnings
  from functools import singledispatch
  from typing import Optional, Tuple, Union
  import numpy
  import pandas
  from sktime.datatypes import VectorizedDF
  from sktime.datatypes._utilities import get_time_index
  from sktime.utils.validation.series import check_time_index, is_integer_index
label_desc: Use image node_id 5 for calling a global function with example usage: infer_freq(y) and returns: None